The present invention relates to a monitoring system for detecting the movement of an object at least partially disposed in a liquid medium, and more particularly relates to a system for monitoring a swimming pool and providing an alarm indication of the presence of a person, animal, or object entering a swimming pool.
The number of backyard swimming pools has been drastically increasing for the past decade, and such pools obviously present a safety hazard to small children, pets, or adults who are unable to swim. Regardless of the safety precautions normally used by adults, instances will arise when the pool is left unattended by qualified rescue persons. Although various pool alarm systems have been devised, such prior art devices have not been widely accepted by the public for a variety of reasons. Evidence of this fact unfortunately is too frequently supported by drownings of unsupervised small children. In many instances, these children could be rescued if an adult in the house were alerted to the fact that the child had entered the pool.
Most public and private pools are fenced to minimize unauthorized use. Teenagers frequently overcome such protection, however, and the unauthorized usage of pools is quite common. Such unauthorized usage obviously increases the likelihood of vandalism to pool equipment. More importantly, such usage has resulted in teenage injury or deaths due to both the increased risks taken by unattended teenagers, and the increased hazards of swimming at night without adequate lights.
One type of prior art pool alarm system is functionally dependent on the physical movement of water which commonly occurs when a person enters the pool. U.S. Pat. No. 3,468,283, for instance, depends on the relative motion between an outer body and a vane member caused by the movement of water. U.S. Pat. No. 3,504,145 includes a buoyant float which moves relative to an outer container in response to wave action within the pool. Similarly, U.S. Pat. No. 3,732,556 includes a sensor circuit closable by the splash of water thereon. U.S. Pat. No. 4,121,200 discloses a sophiscated system including a transducer responsive to air pressure changes in a tube, which in turn is responsive to water displacement information through a collector. Such devices are highly susceptible to false indications and alarm signals (which defeat the security desired), since the alarm may be easily triggered by wind, blowing leaves, or other surface debris. Moreover, these devices are necessarily unreliable since a small child or pet may slowly enter a pool and cause little wave action.
Another type of pool alarm system is responsive to sound waves which are normally created when a person or animal splashes in a pool. Such systems, as shown in U.S. Pat. No. 2,839,915, typically use a hydrophone to receive the person or animal-generated sound waves, and in response thereto generates an electrical signal to an alarm. Similar systems are disclosed in U.S. Pat. Nos. 3,155,954 (with a sensitivity control); U.S. Pat. No. 3,486,166 (with a selected frequency test circuit); and U.S. Pat. No. 4,187,502 (with an omnidirectional hydrophone). A modified system is illustrated in U.S. Pat. No. 3,867,711, with the apparatus being particularly sensitive to acoustic signals for frequency components in the range of the human heartbeat rate.
This second type of apparatus similarly has not been widely accepted in the industry. In order to achieve the desired sensitivity, such units are costly to manufacture and are preferably installed at several places in the pool. These units are similarly susceptible to false indications such as wind, blowing leaves, or miscellaneous ground noise caused by passing cars. Also, these systems again lack reliability because low frequency acoustical energy produced by a person in the pool cannot be easily distinguished for other miscellaneous background noise.
The disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus are hereinafter provided for reliably detecting the movement of persons, animals, or objects in the pool.